The inhibitory action of N,N'-1,3-propylen-bis(3-methoxysalicylidenimine) {PMSI} on mild steel corrosion in sulfuric acid medium was investigated through electrochemical methods and evaluations based on infrared spectroscopy and scanning electron micrographs. The studies revealed that the molecule is a good mixed-type inhibitor (mostly anodic), acts as a multi-dentate ligand and repels the corrosive agents by hydrophobic forces. Its adsorption on metal surface has a physicochemical nature and obeys the Langmuir isotherm. At a critical (optimum) concentration, an anomalous inhibitory behavior was observed and interpreted at microscopic level by means of desorption/adsorption process and horizontal <-> vertical hypothesis. The addition of iodide into acid moreover causes a synergistic influence. a substantial enhancement on inhibitory performance. Finally, using isolated inhibitor calculations at B3LYP/6-31G +(d,p) level of theory, the equilibrium geometry of PMSI was determined and the energy required for hindrance avoidance was predicted. (C) 2010 Elsevier Ltd. All rights reserved.